(1) Field of the Invention
The present invention relates to a seam covered welded can. More particularly, the present invention relates to a seam covered welded can having a covering layer excellent in the corrosion resistance, adhesion and processability on a welded seam, especially a seamed covered welded can having a composite covering layer comprising a specific polyester and a specific copolyester.
(2) Description of the Prior Art
As the process for the production of a can body, there has been widely adopted a process comprising forming a cylindrical body from a metal can blank cut in a predetermined size, lapping both the ends of the blank and bonding the lapped portion by welding or using an adhesive or solder.
In a can body obtained according to this process, a cut end portion of the blank, that is, a cut edge, is inevitably present on the inner face side of the side seam, and in order to prevent corrosion of the blank and dissolution of the metal into a content, it is very important to cover this cut edge of the blank. Especially in case of a welded seam, in addition to the above-mentioned cut edge, the molten metal is exposed on the entire seam, a portion (splash portion) to which the metal protrudes is formed and there also is present a step portion of the seam. Therefore, it is very difficult to make the covering resin layer on the entire surface of the seam.
Various proposals have been made on the process for protecting the seam by forming a covering resin layer on the welded seam. For example, there is known a process in which a solution or powder paint is coated on the inner face side of the seam of a formed can body, or a process in which a thermoplastic resin tape is supplied to the inner face side of the seam and fusion-bonded. Furthermore, there is known a process in which a paint comprising a thermosetting resin and a thermoplastic resin at a certain ratio is applied to the inner face side of a seam to form a protecting coating having a specific dispersion state. However, a paint excellent in the adhesion to the seam tends to be poor in the barrier property to corrosive components while a paint excellent in the barrier property to corrosive components is generally poor in the adhesion. In general, a welded can after covering of the seam is subjected to processing such as necked-in processing, beading, flanging and double seaming and then to retort sterilization at a high temperature exceeding 120.degree. C. Accordingly, if a welded can is poor in any one of adhesion, processability, heat resistance and corrosion resistance, there arises a problem of the dissolution of the metal or the leakage by pitting.
Moreover, in the case where a resin as mentioned above is used, the paint or coating of the resin flows in the molten state so that it fills a stepped portion present in the seam, and therefore, the coating is cut or thinned at an angular part of the cut edge or bubbles are easily contained in the coating at the stepped portion. Accordingly, it is almost impossible to form a complete covering at the cut edge of the blank.